Heroin is deadly enough on its own, but mixed with a powerful painkiller meant for terminally ill cancer patients it creates a potentially even more lethal concoction that could be behind a spike in overdoses in cities and towns south of Boston, police say.

Mixing heroin and the drug fentanyl, a synthetic painkiller 100 times more potent, can cause a user to unknowingly take more than they are used to and end up overdosing, said Detective Lt. Patrick Glynn, head of the Quincy Police Department's drug unit.

"They're assuming it's the heroin they're familiar with but obviously it's being cut with something else," he said. "When (fentanyl) is mixed with heroin, the doses become very pure, and that's what's causing the overdoses."

Glynn said police have used the overdose-reversing drug Narcan, also known as naloxone, to revive 19 overdose victims since Jan. 1. That's about double the number of times as in the same period last year.

Lab reports aren't back yet, but Glynn suspects some of those drug users overdosed on heroin cut with fentanyl.

Three months ago, Frank Johnston, station commander at American Medical Response in Brockton, started noticing that some patients suffering from heroin overdoses needed larger doses of Narcan and that their reaction to the drug was delayed. He believes that's due to heroin mixed with fentanyl.

Johnston said the number of overdoses the ambulance service sees has doubled in recent months.

"In a week, lately we've seen an average of about six to eight overdoses. Normally it's two to four," he said.

Dr. Joseph Shrand, medical director of CASTLE (Clean and Sober Teens Living Empowered), part of the High Point Treatment Center in Brockton, said those who use heroin should be extra cautious about what's out on the street.

"It's a very scary, dangerous thing right now," Shrand said. "When you buy something on the street, it's not FDA-approved. You don't know what the heck is in it."

Dr. David Rosenbloom, professor of health and policy management at Boston University, said mixing heroin and fentanyl is particularly dangerous.

"The problem is that it is so much more powerful than heroin that people who take mixtures of it are at significantly greater risk for overdose," Rosenbloom said. "Fentanyl slows down breathing, and if you have enough of it, then you stop breathing."

In Pennsylvania, the deadly concoction is believed to have been used in at least 17 fatal overdoses in January, according to ABCNews.com. In Maryland, at least 37 deaths were linked to the mixture since September, the website said.

Massachusetts State Police said they are gathering statewide information to figure out how to approach the issue. Lt. Daniel Richard said the department is looking to see if fentanyl is being used in one particular region or statewide. He also said the state drug lab is testing heroin that was seized recently for traces of fentanyl.

Page 2 of 2 - Glynn, the Quincy police detective, said police ran into heroin-fentanyl mixtures 10 to 15 years ago, but the mixtures gradually faded away. Now they seem to be making a comeback.

Marshfield Police Chief Phillip Tavares said he wouldn't be surprised if fentanyl has been behind recent overdoses in his town.

"If it's not already here, I'm sure it will be at some point," he said. "That's a major concern."

Fentanyl is prescribed in numerous fashions – in pill, powder and liquid form. There are fentanyl patches and lollipops.

Scott Peterson, an employee at Learn2Cope, a support group for family members of addicts, said he believes some people are breaking open the patches to expose a jelly-like substance that contains the medicine.

He said the only way people can get fentanyl is with a doctor's prescription.

CASTLE's Shrand urges people who are going to use heroin to not shoot up alone. He said if someone does begin to overdose, to call 911 immediately. He also said that users should keep handy a dose of Narcan – the brand name of the drug Naloxone – in case they overdose.